1. Field of the Invention
The present invention relates to a mobile crane having a counterweight.
2. Description of the Background Art
Heretofore, there has been known a large mobile crane having a counterweight for increasing a lifting capacity. For example, JP 2008-297112 A discloses a mobile crane which comprises a lower propelling body, a upper slewing body adapted to be slewed on the lower propelling body, a lattice-structured mast attached to the upper slewing body, and a counterweight hung from an upper end of the mast through a guy line, at a position rearward of the upper slewing body. In this type of mobile crane, a crane lifting capacity is adjusted by changing a horizontal distance between a mast foot which is a supported point of a base end of the mast and a center of gravity of the counterweight (the horizontal distance will hereinafter be referred to as “hanging radius of the counterweight”), or changing a mass of the counterweight. For example, when a boom is moved frontwardly while hanging a load therefrom (an operating radius is increased), the counterweight is moved rearwardly with respect to the upper slewing body to increase the hanging radius of the counterweight.
The hanging radius of the counterweight is adjusted, for example, by changing an angle of the mast. Specifically, the mast is raised and lowered about the mast foot. The raising and lowering movement of the mast, however, involves a vertical displacement of the mast point at the upper end of the mast, which causes the following problems. When the mast is lowered to increase the hanging radius of the counterweight, the counterweight is displaced downwardly and landed on a ground surface, which hinders the counterweight from functioning as a weight, and further hinders a slewing movement of the upper slewing body and a traveling movement of the mobile crane. On the other hand, when the mast is raised to reduce the hanging radius of the counterweight, the counterweight is lifted up from the ground surface: releasing a load in this state may cause the mobile crane to be inclined rearwardly.
To solving the above problems, it is required to preliminarily calculate/measure the mass of the counterweight, the hanging radius of the counterweight, and a length of the guy line for hanging the counterweight (the guy line will hereinafter be referred to as “counterweight guy line”), on a constant basis. The length adjustment of the counterweight guy line, however, requires a lot of time and effort.
As a mobile crane designed taking into account this point, there has heretofore been known one type equipped with a cylinder for changing a substantial length of the counterweight guy line. There are shown two examples of this type of mobile crane in FIGS. 7 and 8.
FIG. 7 shows a mobile crane 101, which comprises a lower propelling body 10, a upper slewing body 20, a boom 25, a mast 30 having a mast point 30p provided at an upper end thereof, a counterweight 50, and a counterweight guy line 54 for hanging the counterweight 50 from the mast point 30p, wherein a hydraulic cylinder 160 is interposed in an upper portion of the counterweight guy line 54.
The hydraulic cylinder 160 has a cylinder body 162 surrounding an internal space thereof, a piston 164 provided within the cylinder body 162 to partition the internal space into an upper head-side cylinder chamber 160h and a lower rod-side cylinder chamber 160r, and a rod 166 extending from the piston 164 downwardly beyond an lower end of the cylinder body 162. The hydraulic cylinder 160 is operated to be extended/retracted depending on an angle of the mast 30, by means of supply/discharge of a hydraulic pressure to/from the cylinder chambers 160h and 160r, thereby allowing an altitudinal height H of the counterweight 50 from a ground surface G (the altitudinal height will hereinafter be referred to as “counterweight height H”) to be adjusted.
FIG. 8 shows a mobile crane 201 which comprises, instead of the cylinder 160 of the above mobile crane 101, a counterweight support member 241 interconnecting the upper slewing body 20 and the counterweight guy line 54 and a hydraulic cylinder 260 interposed between the counterweight guy line 54 and the counterweight 50. The counterweight support member 241 is adapted to be extended/retracted in a direction close to a horizontal direction, thereby allow a hanging radius R of the counterweight 50 to be changed without moving the mast 30. This type of crane, involving no displacement of the mast point 30p, does not permit a large variation of the counterweight height H; however, if a length of the counterweight guy line 54 was kept constant, the extension/retraction of the counterweight support member 241 could vary the counterweight height H. The hydraulic cylinder 260, which has a structure similar to that of the cylinder 160, is adapted to be extended/retracted in an upward-downward direction to allow the counterweight height H to be adjusted.
However, the above technique, using the hydraulic cylinder 160 or hydraulic cylinder 260 to pull up the counterweight 50 by the retraction of the cylinder, lowers driving efficiency. For example, pulling up the counterweight 50 by use of the hydraulic cylinder 160 shown in FIG. 7 requires a hydraulic fluid to be supplied to the rod-side cylinder chamber 160r on the side of the counterweight 50; in the rod-side cylinder chamber 160r, the piston 164 has a pressure receiving area smaller than that in the head-side cylinder chamber 160h by a cross-sectional area of the rod 166. Therefore, in order to produce a sufficiently large pull-up force despite the relatively small pressure receiving area, it is necessary to increase a hydraulic pressure or increase a diameter of the cylinder 160. Either case requires a costly hydraulic cylinder.